1. Field of the Invention
The invention relates to a process and to a device for measuring the oxygen potential in a silicon melt by means of an electrochemical potential probe which dips into the melt.
2. The Prior Art
In many processes for the production of electronic components with high packing density from silicon wafers, the amount of oxygen contained in the silicon is of great importance. The reason is that, in particular process steps, ever-increasing use is being made of the intrinsic getter action of oxygen. Therefore in microelectronics, monocrystalline silicon is exclusively used which has an oxygen content of about 0 atoms/cm.sup.3.
Monocrystalline silicon is produced by Czochralski crucible pulling. During this pulling process the reaction between the silicon melt and the quartz crucible causes the melt to become enriched with oxygen. Oxygen is also incorporated into the growing crystal ingot. The melt oxygen concentration existing at the crystal growth boundary is also found in the crystal at about the same concentration.
However, the oxygen concentration in the melt changes during the pulling process due to the lowering melt level and the consequent reduction in the contact area between the melt and the quartz crucible. Furthermore, the thermal conditions which essentially influence the reaction between the crucible and the melt are not constant.
According to the prior art, infrared absorption measurements according to ASTM Standard F120/F121 are used to determine the oxygen content of a silicon ingot which has finished being pulled. This measurement is also used to compare this oxygen content with reference values. The parameters which affect the incorporation of oxygen are then corrected for the subsequent pulling process.
Those parameters which may influence the incorporation of oxygen in silicon ingots are the rotation of the crucible and/or of the crystal, the power with which the crucible is heated, the chamber pressure and the throughput of the inert gas flow:
1) the reaction between the silicon melt and the quartz crucible, and therefore the concentration of oxygen dissolved in the melt, can be controlled by increasing or decreasing the rotation of the crucible and/or of the crystal, and the power with which the crucible is heated, PA1 2) the ratio between dissolved oxygen and SiO gas respectively in and above the melt can be controlled by varying the pressure in the chamber, PA1 3) the concentration of SiO gas above the melt can be controlled by means of the throughput of the inert gas flow. PA1 providing an electrochemical potential probe, said probe comprising a SiO.sub.2 glass tube having a closed lower end and containing graphite having an upper end and being in direct contact with the SiO.sub.2 glass tube and said probe having a wire making contact with the graphite at the upper end of the graphite; said contact occurring at a graphite wire contact point; PA1 dipping said probe into the silicon melt only to a level such that graphite wire contact point contained in the SiO.sub.2 glass tube is above the silicon melt; PA1 measuring the voltage between the silicon melt and the wire contacting the probe caused by the difference between the melt oxygen potential and a reference oxygen potential in the probe; and PA1 determining the oxygen potential in the silicon melt from the probe voltage.
These parameters can only be corrected retrospectively, and not be used for a control loop. Thus, it is not possible for a uniform axial and radial oxygen distribution and a defined oxygen content to be achieved either over the entire crystal length or over several crystal ingots.
EP 0 696 652 A2 discloses an electrochemical potential probe for measuring the oxygen potentials in silicon melts. The probe consists of a sealed SiO.sub.2 glass tube which contains a metal/metal oxide mixture and has a metal wire making contact with it. Transition metals, for example, Ti, V, Cr, Mn, Fe, Co and Ni are usually used as the reference mixture (metal/metal oxide).
The diffusion coefficients of these metals in quartz glass are up to 10.sup.-4 cm.sup.2 /s at a temperature of 1420.degree. C. (the melting temperature of Si) (R. Bruckner; Properties and Structure of Vitreous Silica II, J. Non-crystalline Solids 5 (1971) 177).
When potential probes designed in this way are used for a fairly long time (&gt;1 min), increased contamination of the melt by foreign metals must be expected. However, it is desirable to use a potential probe throughout the pulling process. This is because it is possible to make a control loop on the basis of the oxygen concentrations determined by measuring the oxygen potentials and by varying the parameters mentioned above.